Process for producing diethyl phosphite

ABSTRACT

A process of producing diethyl phosphite by reacting an excess of triethyl phosphite with phosphorous acid. The improved process results in a high quality diethyl phosphite product having low acidity that is suitable as an intermediate for most uses requiring a low acidity diethyl phosphite.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the production of diethyl phosphite of highpurity and low acidity.

2. Relevant Prior Art

Diethyl phosphite is useful as an intermediate in the production oforganophosphorous compounds such as flame retardant compounds and asstabilizers for plastics.

U.S. Pat. No. 2,834,797 discloses a method of producing dialkylphosphites by the reaction of trialkyl phosphites with moltenphosphorous acid. The cited patent teaches reacting a trialkyl phosphitewith phosphorous acid in a molar ratio of phosphite to phosphorous acidof about 1:2 to 3:1 and preferably 2:1 for maximum dialkyl phosphiteyields. The diethyl phosphite prepared as disclosed in the cited patentis said to result in a 78% of theory yield after distillation, based ontriethyl phosphite. Additionally, the dialkyl phosphite product producedin accordance with the process of the cited art has been found to havean acid value in excess of 20.2 milligrams KOH per gram, which forcertain applications utilizing diethyl phosphite (for instance, inproducing aminoalkyl phosphonates used as fire retardants in urethanefoams) would reduce the product yield and additionally would decreasethe product purity and prevent it from meeting acidity specification. Insuch applications an acid value of less than 10 mg of KOH per gm ofproduct is desirable. However, an acid value of less than 5 mg of KOHper gram is preferred.

An object of the invention is to produce a diethyl phosphite product ofhigh purity, having low acidity in high yields without requiring furtherdistillation.

SUMMARY OF THE INVENTION

It was unexpectedly discovered that the use of a 0.05-0.30 molar excessof triethyl phosphite over stoichiometric quantities in producingdiethyl phosphite by the reaction of triethyl phosphite with phosphorousacid, significantly increases the product purity and product yield whilesignificantly decreasing the acidity of the diethyl phosphite product.

The invention is a process for producing diethyl phosphite of increasedpurity and low acidity comprising reacting from about 2.05-2.30 ordesirably about 2.08 to about 2.20 moles of triethyl phosphite, per moleof phosphorous acid. The novel process disclosed herein provides adiethyl phosphite product in a yield of about 100% based on phosphorousacid, with purity in excess of 95%, which obviates the need for furtherpurification by distillation; and acidities in the range of 1.5 to 3.5milligrams of KOH per gram of product. Residual triethyl phosphitereactant was found to be in the range of 0.1 to 2.0%.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a process wherein triethyl phosphite is added toanhydrous phosphorous acid, in an amount over stoichiometric quantities,and heated for a period of time to give a low acid, high purity and highyield diethyl phosphite product without the need for furtherpurification treatment as by distillation. The product produced in thismanner is particularly desirable for preparing quality aminoalkylphosphonates utilized as flame retardant compounds in urethane foamproducts.

The following reaction is believed to exemplify the process of producingdiethyl phosphite by reaction of triethyl phosphite and phosphorousacid:

    2P(OC.sub.2 H.sub.5).sub.3 +HPO.sub.3 H.sub.2 →3HPO.sub.3 (C.sub.2 H.sub.5).sub.2

To achieve the low acid, high yield and high purity diethyl phosphiteproduct disclosed herein, about a 2.5-15.0 mole percent excess oftriethyl phosphite, or more desirably, about a 4-10 mole percent excess,is used in accordance with the invention. Preferably, about a 7.5 molepercent excess triethyl phosphite is reacted with the phosphorous acidin accordance with the invention.

As oxygen can cause oxidation of the phosphorous acid to phosphoricacid, and oxidation of the triethyl phosphite to triethyl phosphate,precaution should include avoidance of oxygen during the process.

In addition, since phosphorous acid is hygroscopic and deliquescent, thereactor should be purged with an inert gas, preferably nitrogen, priorto charging the reactants to the reactor. The phosphorous acid isgenerally added to the reactor over a period of time and under an inertgas purge to avoid absorption of water from the atmosphere.

Any gases exiting the reactor generally contain low levels of phosphine,which may be scrubbed from the exiting gas by known methods as, forinstance, scrubbing with dilute sodium hypochlorite to remove thedangerous contaminant.

The anhydrous phosphorous acid can be charged to the reactor with orwithout prior heating of the reactor. Generally, however, the reactorwhich may contain a product heel is heated to 40° C. The mixture, afteraddition of the phosphorous acid is then heated to 80° C. and triethylphosphite is gradually added.

The reaction process is the same with or without utilizing a productheel. It is, therefore, not necessary to utilize a product heel in theact of practicing this invention. If a product heel is utilized, thephosphorous acid and the heel may be mixed with or without theapplication of heat to the reactor prior to dissolving the phosphorousacid. It is desirable, however, to heat the mixture to a temperature offrom about 40°-80° C. prior to adding triethyl phosphite.

In the practice of the invention, the process is conducted at atemperature of from about 60°-150° C., generally at 80°-120° C. andpreferably from 90°-110° C.

At temperatures below 75° C., the reaction is sluggish. Highertemperatures are preferred to avoid increasing the unreacted triethylphosphite content and to increase the reaction rate. Using the amount oftriethyl phosphite over stoichiometric quantities, as disclosed hereincauses complete reaction and advantageously provides a low acid product.Temperatures about 130° C. should be avoided since they can result insome increase decomposition of the acid to phosphoric acid andphosphine. Although some trace amount of phosphine is found under thereaction conditions disclosed herein, this small amount can be scrubbedfrom the exit gas as noted above.

Use of increased amounts of triethyl phosphite over that disclosedherein for reacting with phosphorous acid would not only decrease theacidity of the final product but also decrease the overall purity of thediethyl phosphite product since amounts of unreacted triethyl phosphitewould remain in the product. Similarly, use of stoichiometric quantitiesof the reactants or less than the amount disclosed herein would resultin decreased purity, and increase the acidity of the product.

The reaction is exothermic and can be controlled somewhat by adjustingthe addition rate of triethyl phosphite.

In the preferred process of practicing the invention, phosphorous acidcan be added to the reactor at temperatures of from about 40°-80° C.,particularly where a diethyl phosphite heel is present. The addition oftriethyl phosphite can desirably occur at temperatures of from about80°-130° C. During the triethyl phosphite phosphorous acid reactionperiod, temperatures of about 90°-110° C. are preferred. It should beobvious to one skilled in the art that, utilizing reaction temperaturesat the lower end of the specified range will result in a slower reactionrate and require a longer time period to obtain a product having thedesired acidity level.

The following Examples are illustrative of the invention. Yields ofabout 100% based on phosphorous acid were obtained in all processExamples provided below for the diethyl phosphite product.

EXAMPLE 1

Diethyl phosphite (DEP) was produced by reacting 357.0 grams (2.10moles) of triethyl phosphite in a reactor purged with nitrogen, with 82grams of phosphorous acid (1.0 moles). The triethyl phosphite (TEP)representing a 7.5% excess over stoichiometric quantities was added tothe reactor containing the phosphorous acid and a diethyl phosphite heelof 82 grams at 80° C., over a 30 minute period. During the addition oftriethyl phosphite, the temperature was maintained at from 85°-86° C. bythe rate of addition. Thereafter, the temperature was raised to 95° C.and the reactants were heated over a period of 4 hours to produce adiethyl phosphite product having a 97.9% purity and an acid value of3.34 mg KOH/gm.

EXAMPLE 2

The process of Example 1 was repeated except that the reactants wereheated to a temperature of 110° C., instead of 95° C., after thetriethyl phosphite addition. The purity of the diethyl phosphiteobtained after 5 hours of heating was 97.1% with a KOH acid number of2.24.

EXAMPLE 3

The process of Example 1 was followed using 348.6 grams of triethylphosphite representing a 5% excess. The diethyl phosphite product had apurity of 97.9% after 5 hours of heating at 95° C. and the KOH acidnumber was 4.02.

COMPARATIVE EXAMPLE A

Diethyl phosphite was prepared by reacting triethyl phosphite withphosphorous acid in a molar ratio of 2:1. A solvent heel of 41 grams ofdiethyl phosphite having an acid value of 3.08 mg/KOH/gm. was utilizedin the process. The diethyl phosphite heel was heated to 75° C. prior toadding the phosphorous acid. The triethyl phosphite was added attemperatures between 75°-85° C. over a 30 minute period and thetemperature was then raised to 95° C. The acidity of the productobtained over a five hour reaction period is given below.

    ______________________________________                                        Time at 95° C. (Hrs.)                                                                     Acidity mg/KOH/gm                                          ______________________________________                                        0.5                22.4                                                       2.5                20.5                                                       3.5                20.2                                                       5.0                23.0                                                       ______________________________________                                    

The acidity of the diethyl phosphite product obtained would have beenhigher if a solvent heel possessing high acidity had been used. In orderto achieve acidity levels of <10 mg/KOH/gm., and a purity level inexcess of 95%, the product of the above would have to be distilled whichwould result in some product loss and, therefore, lower the yield ofdiethyl phosphite.

COMPARATIVE EXAMPLE B

Triethyl phosphite was reacted with phosphorous acid in a 2:1 molarratio in a reactor. The reactor was slowly heated to between 70°-95° C.during the reaction period shown below. The following results wereobtained from samples removed over the reaction period.

    ______________________________________                                        Heating Time            Acidity                                               (hrs.)       Temperature                                                                              (mg/KOH/gm)                                           ______________________________________                                        1            70-75°                                                                            23.23                                                 11/2         80-95°                                                                            22.16                                                 21/2         80-95°                                                                            22.16                                                 ______________________________________                                    

EXAMPLE 4

Three batches of diethyl phosphite were prepared in a plant operationusing amounts of reactants as specified below.

    ______________________________________                                                    Batch 1 Batch 2   Batch 3                                         ______________________________________                                        TEP (kgs)     60.46     60.55     60.60                                       DEP Heel (kgs)                                                                              29.5      29.5      29.5                                                                (from     (from                                                               Batch 1)  Batch 2)                                    Phosphorous                                                                   Acid (kgs)    13.9      13.9      13.9                                        % excess TEP   7.5       7.5       7.5                                        ______________________________________                                    

In the process the diethyl phosphite heel was charged to the reactor andheated to 40° C. Phosphorous acid was then added in the amount specifiedand the reactor maintained under a constant nitrogen purge. The reactorcontents were then heated to 80° C. and the triethyl phosphite was addedas the temperature was maintained at between 80°-110° C. Toward the endof the addition of the triethyl phosphite, the temperature wasmaintained between 100°-110° C. and held for a period of 5 hours. Thefollowing results were obtained:

    ______________________________________                                                     Batch 1                                                                              Batch 2    Batch 3                                        ______________________________________                                        Purity wt. %   97.5     96.4       97.2                                       Acidity mg/KOH/g                                                                             2.2      2.4        2.1                                        Specific Gravity                                                                             1.072    1.071      1.070                                      Refractive Index N.sub.D.sup.25                                                              1.407    1.405      1.405                                      Color, APHA    40       40-50      25                                         ______________________________________                                    

What is claimed:
 1. A process for producing diethyl phosphite of lowacidity, high yields and increased purity comprising reacting triethylphosphite with phosphorous acid in a reactor in a molar ratio of fromabout 2.05:1 to about 2.30:1.
 2. The process of claim 1 wherein saidmolar ratio is from about 2.08:1 to about 2.20:1.
 3. The process ofclaim 1 wherein said molar ratio is about 2.15:1.
 4. The process ofclaim 1 or 2 or 3, wherein triethyl phosphite is reacted withphosphorous acid at a temperature of from about 60° to about 150° C. 5.The process of claim 4 wherein triethyl phosphite is reacted withphosphorous acid at a temperature of from about 80° to about 130° C. 6.The process of claim 5 wherein said temperature is from about 90° toabout 110° C.